This invention relates to air flow meters of a hot wire type having a bypass passageway, and more particularly to an air flow meter of the hot wire type suitable for use in measuring the flow rate of air drawn by suction into an internal combustion engine of an automotive vehicle.
A hot wire type air flow meter for measuring the flow rate of air drawn by suction into an internal combustion engine of an automotive vehicle is disclosed in SAE Technical Paper Series 800468, for example. This type of air flow meter comprises bare hot wires suspended in a passageway for a flow of air drawn by suction into the internal combustion engine, to obtain measurements of air flow. Although the bare hot wires offer the advantage of being prompt in response, they suffer the disadvantage that they are liable to be damaged because of the backfire in the air passageway.
To obviate this disadvantage, proposals have been made, for example, in Japanese Patent Laid-Open No. 16259/82, corresponding to DE-OS No. 3019544, to use a hot wire type air flow meter comprising a venturi located in a main passageway for a flow of air drawn by suction into the internal combustion chamber, a bypass passageway connected to the main passageway and allowing a portion of the air flowing through the main passageway to be diverted and flow therethrough as a bypass flow by utilizing the pressure differential between a portion of the main passageway upstream of the venturi and a portion of the main passageway in the vicinity of the narrowest portion of the ventury, and a hot wire type air flow sensor located in the bypass passageway to monitor the bypass flow of air so as to determine the flow rate of all the air drawn by suction into the internal combustion chamber based on the flow velocity sensed by the air flow sensor. The use of the hot wire type air flow meter provided with a bypass passageway has raised the problem that, when the air flow to be measured is a pulsating flow, the hot wires are delayed in response and cause the measurements obtained to become smaller than the actual mean air flow rate. To solve this problem, a proposal has been made, in Japanese Patent Laid-Open No. 135916/83 corresponding to U.S. Ser. No. 461,556, to increase the distance between an inlet portion and an outlet portion of the bypass passageway as compared with the distance obtained by measuring the distance in an axial direction along the main passageway, to compensate for the error in measurements. In this hot wire type air flow meter, a flow of air introduced into the air flow meter is split into two air flows at the inlet portion of the bypass passageway or a junction, so that one air flow proceeds through the main passageway and the other air flow through the bypass passageway until the two air flows join at the outlet portion of the bypass passageway or a confluence and flow downwardly out of the air flow meter. The flow velocity of the air flow through the bypass passageway is measured by a hot wire probe and a temperature probe, and the relation between air velocity signals produced by the air flow sensor which senses the air flow velocity through the bypass passageway and the flow rate of all the air flowing through the air flow meter is set beforehand, to allow the flow rate of all the air to be determined based on the flow velocity signals produced by the air flow sensor located in the bypass passageway. In order to ensure that the determination of the flow rate of all the air flowing through the air flow meter is achieved with a high degree of accuracy and precision, the ratio of the air flow through the bypass passageway to the air flow through the main passageway should remain constant at all times, even if the flow rate of all the air flowing through the air flow meter undergoes fluctuations.
However, this hot wire type air flow meter is not without a disadvantage. When the wall of the air flow meter is heated, the flow rate of air flow through the bypass passageway becomes lower than the flow rate of air flow through the main passageway, with the result that the air in the bypass passageway becomes higher in temperature than the air in the main passageway. As a result, the air in the bypass passageway is expanded and the resistance offered by the passageway to the air flow increases, causing the flow rate of air through the bypass passageway to become relatively lower than the flow rate of air through the main passageway. Thus, the hot wire type air flow meter of the aforesaid construction suffers the disadvantage that heating or cooling the wall of the air flow meter causes a drop in the accuracy of the value of an air flow determined by the air flow meter.
In the hot wire type air flow meter of the aforesaid construction, a portion of the air flow through the main passageway which is located near the wall of the main passageway is introduced into the bypass passageway and sensed by the air flow sensor. Generally, a portion of an air flow through an air passageway which is located near the wall of the air passageway tends to become more turbulent than a portion flowing through the center of the air passageway. Thus, signals produced by the hot wire type air flow sensor that monitors the air flow through the bypass passageway which is constituted by the portion of the air flow through the main passageway which is high in turbulence would have a high noise to signal ratio (N/S ratio).